LT3433 LINER [Linear Technology], LT3433 Datasheet - Page 5

• Manufacturer Part Number: LT3433
• Description: High Voltage Step-Up/Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

PI FU CTIO S
SGND (Pins 1, 8, 9, 16): Low Noise Ground Reference.
V
ply rail is referenced to the SW_H pin. Supply voltage is
maintained by a bootstrap capacitor tied from the V
to the SW_H pin. A 1 F capacitor is generally adequate for
most applications.
The charge on the bootstrap capacitor is refreshed through
a diode, typically connected from the converter output
(V
operation assures that the boost capacitor is refreshed each
switch cycle. The LT3433 supports operational V
ply voltages up to 75V (absolute maximum) as referenced
to ground.
SW_H (Pin 3): Boosted Switch Output. This is the current
return for the boosted switch and corresponds to the emitter
of the switch transistor. The boosted switch shorts the
SW_H pin to the V
cuitry for this switch is boosted above the V
through the V
maximum efficiency. The “ON” resistance of the boosted
switch is 0.8 .
V
to the boosted switch and corresponds to the collector of
the switch transistor.This pin also supplies power to most
of the IC’s internal circuitry if the V
externally. This supply will be subject to high switching
transient currents so this pin requires a high quality bypass
capacitor that meets whatever application-specific input
ripple current requirements exist. See Applications Infor-
mation.
BURST_EN (Pin 5): Burst Mode Enable Pin. Shorting this
pin to SGND enables Burst Mode operation. If Burst Mode
operation is not desired, connecting this pin to V
will disable the burst function.
V
pin corresponds to the maximum switch current per oscil-
lator cycle. The error amplifier is typically configured as an
integrator circuit by connecting an RC network from this
pin to ground. This circuit typically creates the dominant
pole for the converter regulation feedback loop. Specific in-
tegrator characteristics can be configured to optimize tran-
sient response. See Applications Information.
BST
IN
C
OUT
U
(Pin 6): Error Amplifier Output. The voltage on the V
(Pin 4): Input Power Supply. This pin supplies power
(Pin 2): Boosted Switch Supply. This “boosted” sup-
), during the switch-off period. Minimum off-time
U
BST
pin, allowing saturation of the switch for
IN
U
supply when enabled. The drive cir-
BIAS
pin is not driven
BIAS
IN
BST
or V
supply
BST
sup-
OUT
pin
C
V
ing input of the error amplifier is connected to an internal
1.231V reference. The V
divider from the converter output. Values for the resistor
connected from V
nected from V
gram converter output voltage (V
relation:
The V
high value feedback resistors could cause a converter
output that is slightly higher than expected. Bias current
error at the output can be estimated as:
The voltage on V
frequency through a “frequency-foldback” function. When
the V
than the 200kHz typical operating frequency. The oscilla-
tor frequency slows with reduced voltage on the pin, down
to 50kHz when V
The V
through a “current-limit foldback” function. At V
maximum switch current is reduced to half of the normal
value. The current limit value increases linearly until V
reaches 0.6V when the normal maximum switch current
level is restored. The frequency and current-limit foldback
functions add robustness to short-circuit protection and
help prevent inductor current runaway during start-up.
SS (Pin 10): Soft Start. Connect a capacitor (C
pin to ground. The output voltage of the LT3433 error
amplifier corresponds to the peak current sense amplifier
output detected before resetting the switch output(s). The
soft-start circuit forces the error amplifier output to a zero
peak current for start-up. A 5 A current is forced from the
SS pin onto an external capacitor. As the SS pin voltage
ramps up, so does the LT3433 internally sensed peak cur-
rent limit. This forces the converter output current to ramp
from zero until normal output regulation is achieved. This
function reduces output overshoot on converter start-up.
FB
V
(Pin 7): Error Amplifier Inverting Input. The noninvert-
OUT
V
FB
FB
OUT(BIAS)
FB
pin voltage is below 0.8V, the oscillator runs slower
pin input bias current is 35nA, so use of extremely
= 1.231 • (R
pin voltage also controls switch current limit
FB
= 35nA • R
to ground (R
FB
FB
OUT
= 0V.
also controls the LT3433 oscillator
FB1
to V
+ R
FB
FB
FB1
pin is connected to a resistor
FB2
FB2
(R
)/R
FB1
) can be calculated to pro-
FB2
) and the resistor con-
OUT
) via the following
LT3433
SS
FB
) from this
= 0V, the
3433ia
5
FB